Real-time, cross-correlating millimeter wave imaging system using dual pill-box antennas

ABSTRACT

A method and apparatus are disclosed for forming an image from millimeter waves. A field of view scanned using two geometrically orthogonal, intersecting copolarized fan beams ( 110, 120 ) to receive millimeter wave radiation. The received millimeter wave radiation from said fan beams are then cross-correlated ( 250, 650 ). Also, a method and antenna ( 400, 610 ) for receiving millimeter wave radiation are disclosed. The antenna includes first and second fan beam antennas ( 410, 420 ) for receiving millimeter wave radiation and a filter ( 430, 440 ) for rotating polarization of incident millimeter wave radiation through 90 degrees received by the second fan beam antenna ( 410 ). The respective first and second beams ( 110, 120 ) intersect and are co-polarized and geometrically orthogonal to each other. Still further, a millimeter wave imaging system ( 600 ) and method are also disclosed, which utilise an antenna ( 610 ) for receiving millimeter wave radiation, process the received millimeter wave radiation from the antenna ( 610 ), and build up the image ( 682 ) using a filtered, cross-correlated signal.

FIELD OF THE INVENTION

The present invention relates generally to millimeter imaging systemsand in particular to a realtime millimeter imaging system for detectingmillimeter wave radiation and generating a corresponding image.

BACKGROUND

Millimeter-wave imaging systems produce a picture of a scene bydetecting thermally generated radiation in the 30-300 GHz range, whichis emitted or reflected by objects in the field of view of theinstrument. Such systems offer advantages over equivalent instrumentsdetecting infrared and visible light, because the millimeter-waveradiation can penetrate low visibility and obscuring conditions (e.g.,caused by clothing, walls, clouds, fog, haze, rain, dust, smoke,sandstorms) without the high level of attenuation that occurs at theother noted wavelengths. This is particularly the case in specific“windows” for atmospheric transmission of radio waves that occur between90 and 110 GHz and between 210 and 250 GHz.

Millimeter-wave imaging systems may be used in a range of importantapplications such as: aids to aircraft landing; collision warning inair, land and sea transport; detection and tracking of ground basedvehicular traffic; covert surveillance for intruders, contraband andweapons. In such applications, the availability of real-time,“movie-camera” like imaging is highly desirable. However, for suchsystems to find wide acceptance in the commercial market-place, thesensing instrumentation must be light in weight, small in size, andaffordable in cost.

A range of millimeter-wave imaging systems have been reported, but failto meet the size, weight, and cost requirements for wide commercialacceptance of the technology, while at the same time offering real-timemoving images. Such systems use two distinct technologies: mechanicalscanning of the beam of a single antenna, and two-dimensional arrays.

Mechanical scanning of the beam of a single antenna connected to asingle receiving system is performed in a raster pattern over a scene todetect the emitted radiation and produce a map or image of thebrightness. The angular resolution of the resultant image is determinedby the width of the antenna beam, whereas the scan angle determines thefield of view. Rapid real-time imaging is difficult or inadequate,because physically large and cumbersome antenna elements (required toachieve high angular resolution) must be moved quickly at high rates.

Two-dimensional arrays of electrically-small antennas and integratedreceivers sample the magnitude of the received millimeter-wave signal atthe focal plane of an antenna system. This information is then used toproduce a snap-shot of the brightness in the field of view of theinstrument. In any given plane, the angular resolution of the resultantimage is determined by the number of elements across the array and theouter dimensions of the array. In contrast, the field of view isdetermined by the beam-width of the individual antenna-array elements.Rapid real-time imaging can be achieved with these systems. However,this occurs at the expense of large numbers (1000's) of millimeter-wavereceiving sub-systems and complex electronic phase shifting andamplitude weighting networks. Because of the large number of receiversrequired, heterodyne systems are avoided (in view of the localoscillator distribution problems) in favour of direct detection systems,with the attendant problems of gain stability and poorer sensitivity.Coherent local oscillator distribution to such a large number ofmillimeter-wave heterodyne receivers presents significant difficulties.

Thus, a need clearly exists for an improved real-time millimeter-waveimaging system capable of producing real-time, movie-like imaging, inwhich the system is more compact, less complex, and less expensive toproduce.

SUMMARY

In accordance with a first aspect of the invention, an image is formedfrom millimeter waves. To do so, a field of view is scanned using twogeometrically orthogonal, intersecting co-polarized fan beams to receivemillimeter-wave radiation. The components of received millimeter-waveradiation from the two fan beams are cross-correlated. The polarizationsof the electric fields of the two fan beams are arranged to besubstantially parallel in alignment. This may be achieved bypolarization rotation filtering of the millimeter-wave radiationreceived in one of the fan beams. The two fan beams may be scanned inazimuth and elevation defining a scan range. The intersection region ofthe two fan beams is able to cover any point in the scan range. The scanrange determines the field of view and a beam width of each fan beam inthe narrow direction determines an angular resolution of the image. Thecross-correlated output is measured at each point in the field of viewto produce a map of the brightness. The position of the twogeometrically orthogonal, intersecting fan beams may be controlled togenerate the cross-correlated output at each fan beam intersection pointin the field of view. Preferably, the scanning is implemented using adual fan-beam antenna The dual fan-beam antenna may have two modifiedpill-box antennas and a polarization rotator to change the direction ofthe incident polarization for one of the modified pill-box antennas. Animage may be formed from millimeter waves of a different polarization byhaving a polarization rotator to change the direction of the incidentpolarization for a different modified pill-box antenna, only onepolarization rotator being used at any time.

In accordance with a second aspect of the invention, millimeter-waveradiation is received. A field of view is scanned using a fan beam toreceive millimeter-wave radiation. Polarization of incidentmillimeter-wave radiation is rotated through 90 degrees, and the fieldof view is scanned using another fan beam to receive thepolarization-rotated millimeter-wave radiation. The fan beams intersectand are geometrically orthogonal to each other, yet the radiation isco-polarized. The fan beams are provided by respective fan-beamantennas. Each such antenna may include a modified pill-box antenna.Preferably, the modified pill-box antenna includes: a metal housing withan elongated aperture in at least one side of the housing, a curvedprimary reflector surface located within the housing and opposite theaperture, a feed horn within the housing, and one or more sub-reflectorsfor coupling the feed horn to the primary reflector surface. At leastone of the sub-reflectors is designed to rotate, providingone-dimensional beam scanning in the narrow direction of the fan beam.The polarization rotation for a fan beam may be implemented using apolarization rotating transreflector.

Preferably, the transreflector includes: a planar metallic reflector,and a grid of closely spaced wires. The wires are preferably spacedn×λ/4 from the planar metallic reflector, where n is an odd integer andλ is a wavelength of the millimeter-wave radiation. The polarizationrotating transreflector may be positioned at a 45 degree angle relativeto the aperture of the second fan-beam antenna and at a substantially 45degree angle relative to the direction of incident millimeter-waveradiation. The polarization rotation for a fan beam may be switched byexchanging a polarization rotating transreflector and a planar metallicreflector, both aligned in the same way. An exchange may be effected byturning a polarization rotating transreflector by 180 degrees to use itsback surface as a planar metallic reflector. An exchange may be effectedby making the wires of a polarization rotating transreflector out of amaterial that has a switchable conductivity.

In accordance with a third aspect of the invention, millimeter waveradiation is received for generating an image. To do so, millimeter waveradiation is received in accordance with first and second fan beams. Thefirst and second fan beams are geometrically orthogonal to each otherand intersecting. The millimeter wave radiation received in accordancewith the second fan beam is co-polarized with the millimeter waveradiation received in accordance with the first fan beam. Components ofthe millimeter wave radiation received in accordance with the first andsecond beams are downconverted to generate respective intermediatefrequency (IF) signals. The IF signals are cross-correlated. Theresulting cross-correlated signal is filtered to provide a valueproportional to brightness at each point in the scene. The receivedmillimeter wave radiation may be amplified in accordance with the firstand second beams prior to the step of downconverting.

In accordance with a fourth aspect of the invention, millimeter-waveimaging is disclosed. To do so, millimeter-wave radiation is received.The receiving includes: receiving millimeter-wave radiation by scanninga field of view using a fan beam, rotating the polarization of incidentmillimeter-wave radiation through 90 degrees, and receiving thepolarization-rotated millimeter-wave radiation by scanning a field ofview using another fan beam. The fan beams intersect and aregeometrically orthogonal to each other. The received millimeter-waveradiation is processed. The processing step includes: receivingcomponents of millimeter-wave radiation from the antenna received inaccordance with the fan beams, downconverting respective components ofthe received millimeter wave radiation received to generate respectiveintermediate frequency (IF) signals, cross-correlating the IF signals;and filtering the resulting cross-correlated signal. The filtered,cross-correlated signal is proportional to the brightness at each pointin the field of view as the antenna beams are scanned. In this way, animage of the scene may be built up. The scanning of each fan beam may beindependently controlled as required so that the image can be generatedfrom the filtered, cross-correlated output signal which provides a valueproportional to the brightness of the scene at each point in said fieldof view.

BRIEF DESCRIPTION OF THE DRAWINGS

A small number of embodiments are described hereinafter with referenceto the drawings, in which:

FIG. 1 is a radiation pattern of two crossed fan beam antennas inaccordance with the embodiments of the invention;

FIG. 2 is a simplified block diagram of a real-time millimeter-waveimaging system in accordance with an embodiment of the invention;

FIG. 3 is a perspective view of an example of a pill-box antenna forimplementing a scanned-beam imaging system in accordance with anotherembodiment of the invention;

FIG. 4 is a perspective view of a combination of two pill-box antennasand a metallic reflector for producing a dual-scanning beam antenna withco-polarized far-field response in accordance with a further embodimentof the invention;

FIG. 5 is a perspective view of a combination of two pill-box antennasand two polarization rotating transreflectors that may be exchanged forplanar metallic reflectors, for producing a dual-scanning beam antennawith co-polarized far-field response of either of two polarizations, inaccordance with a further embodiment of the invention; and

FIG. 6 is a block diagram illustrating a real-time cross-correlatingmillimeter-wave imaging system in accordance with a further embodimentof the invention, incorporating the dual fan-beam antenna of FIG. 4 orFIG. 5 in a modified millimeter-wave imaging system of FIG. 2.

DETAILED DESCRIPTION

A method and an apparatus for forming an image from millimeter waves, amethod and an antenna for receiving millimeter wave radiation, a methodand an apparatus for receiving millimeter wave radiation for generatingan image, and a method and a system for millimeter wave imaging aredisclosed. In the following description, numerous specific details areset forth. In the other instances, details well known to those skilledin the art may not be set out so as not to obscure the invention. Itwill be apparent to those skilled in the art in the view of thisdisclosure that modifications, substitutions and/or changes may be madewithout departing from the scope and spirit of the invention.

The embodiments of the invention involve improved imaging methods,antennas, and systems that enable the realization of a simple, low-costinstrument, capable of realtime imaging of moving targets. In broadterms, the embodiments produce a map or image of the millimeter-wavebrightness in the field of view of the instrument by cross-correlatingthe signal received from two orthogonal, intersecting fan-beams.

Fan-Beam Antennas Generally

An antenna with a fan-beam radiation pattern detects radiation from aregion in the field of view that is of narrow angular extent in onedirection only, while possessing a broad pattern in the orthogonalplane. Typically, a fan-beam can be generated by an antenna, or array ofantennas, which is essentially one-dimensional (e.g., a long narrowslot, a linear array of slots, or a linear array of patch antennas). Thewidth of the beam in the narrow direction is inversely proportional tothe electrical length of the aperture or array. In contrast, thebeam-width in the broad direction is inversely proportional to the widthof the aperture or an individual element of the array. The angularposition of the fan-beam in the narrow direction may be scanned acrossthe field of view by producing a varying linear gradient in the phase ofthe electrical excitation across the aperture or across the elements ofthe array.

In accordance with embodiments of the invention, two such fan beams arearranged so that the beams intersect at right angles in the field ofview of the instrument. FIG. 1 is a plot illustrating the radiationpattern 100 of two crossed fan beam antennas. The pattern 100 includesan E-plane, fan-beam antenna pattern 110 and an H-plane, fan-beamantenna pattern 120, and a pencil beam pattern 130. The polarization ofthe electric field in each beam is arranged to be parallel in alignment.As the fan-beams 110, 120 are scanned in azimuth and elevation, theintersection region 130 can be made to cover any point in the scanrange. Thus, the scan range determines the field of view of theinstrument and the beam-width of the fan-beam in the narrow directiondetermines the angular resolution of the image. The millimeter-wavebrightness at any point in the image is proportional to thecross-correlation between the signals received by the two antennasystems.

Imaging Receiver System

A significant component of the imaging system is the receiver, whichtakes the output from the antennas, amplifies the signals, and thendown-converts the amplified signals to a convenient intermediatefrequency at which the cross-correlation can take place. There are anumber of possible implementations for such receiving systems, dependingupon the design of the fan-beam antenna.

An imaging receiver system 200 in accordance with an embodiment of theinvention shown in FIG. 2 uses only two receivers, one connected to anantenna 202 scanning in the vertical direction and the other to anantenna 204 scanning in the horizontal plane, to sample the whole image.The antenna 202 is an E-plane antenna, and the antenna 204 is an H-planeantenna. The E-plane antenna is coupled to one or more radio frequency(RF) low noise amplifiers (LNAs) 212 a, 212 b. The output of the one ormore low noise amplifiers 212 b is coupled to a respective block downconverter 232. Similarly, the H-plane antenna 204 is coupled to one ormore LNAs 214 a, 214 b. The output of the LNA 214 b is coupled to afurther block down converter 234. A local oscillator 220 provides aninput to both block down converters 232, 234.

The respective block down converters 232, 234 produce respectiveintermediate frequency (IF) signals that are both provided to acorrelator 240. The output of the correlator 240 is provided to a lowpass filter 250, which produces the output signal 260. A map of themillimeter-wave brightness at each point in the field of view isproduced by scanning the antenna beams over the field and at each fieldpoint measuring the cross correlation between the receiver outputs usinga broadband analogue multiplier 240.

A polarization rotating filter (not shown) may be placed in front of oneof the antenna apertures so that both fan beams operate in the samepolarization.

Antenna for Imaging System

In accordance with an embodiment of the invention, a simple, inexpensiveimplementation uses a multiple reflector “pill-box” style antenna 300shown in FIG. 3. In this simplified example, a shaped primary reflector334 is coupled to a single feed-horn 330, 332 via a rotatingsub-reflector 320, which provides beam scanning as the sub-reflector 320spins. More than one sub-reflector may be practiced, with at least onesub-reflector rotating to provide beam scanning. With careful mechanicaland electrical design, in which the rotating sub-reflector 320 rotatesabout its center of mass, high speed scanning can be achieved.Preferably, the sub-reflector 320 is disc-like in form. A significantadvantage of this system is that only a single heterodyne receiver perbeam is needed. This is advantageous from the point of view of systemsimplicity and cost and also because a simple local oscillatordistribution system is possible without the need for complex arrayphasing.

In a conventional “pill-box” antenna, a parabolic cylinder is used asthe reflector. The “pill-box” is formed by two parallel planes which cutthrough the parabolic cylinder perpendicular to the cylinder elements.Typically, the focal line of the cylinder is positioned in the center ofthe aperture formed by the open ends of the parallel plates. When a feedhorn is placed at the focal line, the feed horn blocks a significantportion of the aperture, resulting in large sidelobes in the far-fieldpattern of the antenna as well as standing waves within the “pill-box”itself.

Much improved performance can be obtained when an offset feedingarrangement is used, so that only one side of the “pill-box” isilluminated. The arc of the parabola does not include its vertex, andthe feed horn points to illuminate this arc. Even though theillumination is asymmetric, good sidelobe performance is obtained.Alternatively, the “pill-box” antenna may be symmetrical about the axisof the parabola, but arranged as a folded lens to avoid blockage. Suchan antenna, however, is more difficult to manufacture than an unfoldeddesign.

The millimeter-wave fan-beam antenna 300 shown in FIG. 3 includes ametal housing 310 with a radiating aperture 312 formed in one side ofthe metal housing. The length of the radiating aperture 312 isapproximately 200 wavelengths (λ) and the width of the aperture 312 isapproximately one wave length (1λ). These measurements are preferred andother dimensions may be practiced without departing from the scope andspirit of the invention. The direction of the electric field at theaperture is indicated by an arrow 314. Located within the metal housing310 is the primary reflector surface 334 coupled to the tapered waveguide feed-horn 330 with a wave guide input/output 332 oppositelypositioned relative to the radiating aperture 312 within the housing310. At the bottom of the tapered wave guide feed-horn 330 within themetal housing 310 is the rotating sub-reflector 320 for one dimensionalbeam scanning.

The antenna 300 uses one or more sub-reflectors 320 to couple the feedhorn 330, 332 in an offset “pill-box” structure. The primary reflector334 is shaped away from the traditional parabola to provide enhancedoff-axis scanning angle with good sidelobe performance over the widestpossible range of scan. The primary reflector 334 is coupled to thesingle feed-horn 330 via one or more sub-reflectors 320, which are alsodesigned to have a profile that enhances the scan performance of thecomplete antenna assembly 300. One of these secondary mirrors 320 isarranged so that this sub-reflector 320 rotates, providing main beamscanning as the sub-reflector 320 spins. With careful mechanical andelectrical design, in which the rotating sub-reflector 320 rotates aboutits center of mass, high speed scanning can be achieved.

For the imaging system, a pair of independently-scanned,orthogonally-oriented fan beams are required, with the sense of electricpolarization aligned in each beam. Two “pill-box” antennas 410, 420 ofthe type shown in FIG. 3 are used, configured 400 as shown in FIG. 4.The antenna 410 has an aperture 414 oriented lengthwise in a horizontalsense, while the other antenna 420 has an aperture 424 lengthwise in avertical sense, as depicted in FIG. 4. The direction 412, 422 of theelectric field in the respective apertures 414, 424 are shown. Thus, theaperture 424 couples directly to the observed scene, while the otheraperture 414 is arranged at a right angle so that the aperture 414 iscoupled via a passive reflecting screen 430, 440 and is oriented so thatthe narrow dimension of the far-field pattern of the aperture 414 is atright angles to the pattern of the other antenna 420.

The passive reflecting screen 430, 440 is generally configured at anangle of 45° relative the surface of the fan-beam antenna 410 having theaperture 414. The passive reflecting screen preferably has a planarmetallic reflector 430 spaced apart by a multiple of a quarterwavelength (nλ/4) from a closely spaced, fine wire grid 440. The grid440 is located between the reflector 430 and the antenna 410. The wiresof the grid 440 are aligned at 45° to the direction of incident fieldpolarization. This arrangement 400 results in orthogonal polarization inthe far-field, if a standard plane reflector 430 is used.

Another way to achieve a co-polarized far-field response may be tomodify the feed for the “pill-box” antenna 410, 420, so that the E-fieldvector is rotated through 90 degrees and aligned parallel to the longdirection of the aperture. For this configuration, small variations inthe surface quality and spacing of the metallic walls may causesignificant degradation in antenna performance. However, for thisarrangement, the polarization rotating filter 430, 440 is no longerrequired to be included.

The preferred way to achieve co-polarization is by the use of a“transreflector” 430, 440. The transreflector 430, 440 consists of thewire grid 440, with wires aligned at 45 degrees to the incident electricfield vector, backed by the planar metallic mirror 430 spaced away by anodd-multiple of a quarter wavelength at the operating frequency. Thewire spacing and wire diameter must both be small compared to theoperating wavelength. Over a limited bandwidth determined by the spacingbetween the grid 440 and the reflector 430 (the higher the number ofquarter wavelengths, the narrower the bandwidth), this arrangementresults in a rotation of the polarization of the incident wave through90 degrees, without significantly altering the far-field radiationpattern of the antenna system.

Two “pill-box” antennas 510, 520 of the type shown in FIG. 3 configured500 in an alternative manner are shown in FIG. 5. Generally, FIG. 5shows how two pill-box antennas can be placed with their flat sidesparallel and the apertures oriented 90 degrees apart. In front of bothapertures is a polarization rotating transreflector that can beexchanged with a planar metallic reflector, such that only one aperturereceives polarization-rotated radiation at any time. This leads to amore compact structure than FIG. 4 that is capable of forming an imageof either of two polarizations. The axes of the rotating sub-reflectorsare parallel, so a simple gearing mechanism can be used to give therelative rotation rates needed for the intersection of the fan beams toperform a raster scan.

The antenna 510 has an aperture 530 oriented lengthwise in a horizontalsense, while the other antenna 520 has an aperture 540 orientedlengthwise in a vertical sense, as depicted in FIG. 5. In front of bothapertures is a polarization rotating transreflector that can beexchanged with a planar metallic reflector, such that only one aperturereceives polarization-rotated radiation at any time. In FIG. 5 thehorizontal aperture 530 is coupled to the observed scene via atransreflector 550, while the vertical aperture 540 is coupled to theobserved scene via a planar metallic reflector 560. The transreflector550 may be exchanged with a planar metallic reflector, and the planarmetallic reflector 560 may be exchanged with a transreflector, asindicated by the dotted lines on the reflector 560. An exchange may beeffected by turning a polarization rotating transreflector by 180degrees to use its back surface as a planar metallic reflector. Anexchange may be effected by making the wires of a polarization rotatingtransreflector out of a material that has a switchable conductivity. Theadvantages of this configuration 500 over the configuration 400 in FIG.4 are that the configuration 500 occupies a smaller overall volume andis capable of forming an image from either of two polarizations. Theaxes of the rotating sub-reflectors 320 are parallel in thisconfiguration 500, so a simple gearing mechanism (not shown) can be usedto achieve relative rotation rates that cause the intersection 130 ofthe fan beams 110, 120 to perform a raster scan of the field of view.

FIG. 6 is a block diagram illustrating an implementation of a real-time,cross-correlating, millimeter-wave imaging system 600 in accordance witha further embodiment of the invention. For purposes of illustrationonly, the system is shown in FIG. 6 with a tree 602 as the object ofimaging in the field of view. A dual, fan-beam antenna 610 is used toscan the object 602 and respective horizontal and vertical scans 604,606 generated by the antenna 610 are shown. The dual fan-beam antenna610 is of the type 400 shown in FIG. 4. Alternatively, the dual fan-beamantenna 610 may be of the type 500 shown in FIG. 5. The dual fan-beamantenna 610 provides respective E-plane and H-plane outputs to animaging receiver system, similar to that shown in FIG. 2.

The E-plane output is provided to a low noise amplifier 612 and theH-plane output is provided to a different low noise amplifier 614. Inturn, the low noise amplifiers 612, 614, acting as RF amplifiers, arecoupled to respective mixers 620, 622. Further, a local oscillator 630is coupled to both of mixers 620 and 622. The respective outputs ofmixers 620 and 622 are provided as inputs to IF amplifiers 640, 642. Theoutput of the IF amplifiers 640, 642 are provided to a cross-correlator652.

The output of the cross-correlator 652 is provided to a base band filter660. The base band filter 660 provides the output signal for the system.The output of the base band filter 660 is provided to an analogue todigital (A/D or ADC) converter 670. The ADC 670 produces digital datafrom the output signal that is provided as input to a computer 680. Thecomputer 680 using hardware and/or software can produce a computer image682 using the digital data from the ADC 670. In turn, using the digitaldata, the computer 680 can provide scan control signals 690 (indicatedby dashed lines) to the dual fan-beam antenna 610. As shown in FIG. 6,the scan control signals 690 are preferably provided to each of thepill-box antennas.

The embodiments of the invention have various advantages including oneor more of:

Use of a “pill-box” antenna to implement a scanned-beam imaging system;

A “pill-box” antenna in which the beam is scanned in one dimension usinga rotating sub-reflector;

Use of a wire-grid transreflector to achieve a dual-scanning-beam systemwith co-polarized far-field response;

Use of two wire-grid transreflectors, exchangeable for planar metallicreflectors, to achieve switchable polarisation of the far-fieldresponse.

Use of a mechanically scanned beam so that only a single heterodynereceiver per beam is needed.

Use of two intersecting fan beams so that each antenna is required toscan only in one direction.

Thus, a method and an apparatus for forming an image from millimeterwaves, a method and an antenna for receiving millimeter wave radiation,a method and an apparatus for receiving millimeter wave radiation forgenerating an image, and a method and system for millimeter wave imaginghave been disclosed. In the light of this disclosure, it will beapparent to those skilled in the art that modifications, substitutionsand/or changes may be made without departing from the scope and spiritof the invention.

1. A method of receiving millimeter wave radiation, said methodincluding the steps of: scanning a field of view using a fan beam toreceive millimeter wave radiation; rotating polarization of incidentmillimeter wave radiation through 90 degrees using at least onepolarization rotating transflector; and scanning said field of viewusing another fan beam to receive said polarization-rotated millimeterwave radiation, said fan beams intersecting and being geometricallyorthogonal to each other, and said radiation being co-polarized, saidfan beams are provided by respective fan beam antennas, each fan beamantenna being a modified pill-box antenna including: a metal housingwith an elongated aperture in at least one side of said housing; acurved primary reflector surface located within said housing andopposite said aperture; a feed horn within said housing; and one or moresub-reflectors for coupling said feed horn to said primary reflectorsurface, at least one of said sub-reflectors being designed to rotateand providing one-dimensional beam scanning in a narrow direction ofsaid other fan beam.
 2. The method according to claim 1, wherein saidpolarization rotating transreflector includes: a planar metallicreflector; and a grid of closely spaced wires, said grid spaced n×λ/4from said planar metallic reflector, where n is an odd integer and λ isa wavelength of said millimeter wave radiation.
 3. The method accordingto claim 2, wherein said polarization rotating transreflector ispositioned at a 45 degree angle relative to an aperture of said otherfan beam antenna and at substantially 45 degrees relative to thedirection of incident millimeter-wave radiation.
 4. The method accordingto claim 1, further comprising the step of forming an image from saidreceived millimeter wave.
 5. The method according to claim 4, whereinpolarizations of the electric fields of said two fan beams aresubstantially parallel in alignment.
 6. The method according to claim 5,wherein said millimeter wave radiation received in one of said fan beamsis polarization rotation filtered.
 7. The method according to claim 4,wherein said scanning steps are performed in azimuth and elevationdefining a scan range, and an intersection region of said two fan beamsis able to cover any point in said scan range.
 8. The method accordingto claim 7, wherein said scan range determines said field of view and abeam width of each fan-beam in a narrow direction determines an angularresolution of said image.
 9. The method according to claim 4, furtherincluding the step of measuring cross-correlated output dependent uponsaid received millimeter wave radiation at each field point in saidfield of view to produce a map of brightness.
 10. The method accordingto claim 9, further including the step of controlling said twogeometrically orthogonal, intersecting fan beams to generate saidcross-correlated output at each fan beam intersection point in saidfield of view.
 11. The method according to claim 4, wherein apolarization rotator changes the direction of incident polarization forone of said modified pill-box antennas.
 12. The method according toclaim 1, further including the steps of: downconverting components ofsaid millimeter wave radiation received in accordance with said beams togenerate respective intermediate frequency (IF) signals;cross-correlating said IF signals; and filtering the resultingcross-correlated signal to provide a value proportional to brightness ateach point in the scene.
 13. The method according to claim 12, furtherincluding the step of amplifying said received millimeter wave radiationin accordance with said beams prior to said step of downconverting. 14.The method according to claim 1, further comprising the steps of:processing said received millimeter wave radiation, said processing stepincluding: receiving components of millimeter wave radiation from anantenna received in accordance with said fan beams; downconvertingrespective components of said received millimeter wave radiationreceived to generate respective intermediate frequency (IF) signals;cross-correlating said IF signals; and filtering the resultingcross-correlated signal to produce a filtered, cross-correlated signalproportional to brightness at each point in said field of view as saidantenna beams are scanned; and building up an image using said filtered,cross-correlated signal.
 15. The method according to claim 14, furtherincluding the step of independently controlling the scanning of saidantenna, so that said image can be generated from said filtered,cross-correlated signal, which provides a value proportional to thebrightness of the scene at each point in the field of view.
 16. Anantenna for receiving millimeter wave radiation, said antenna including:a fan beam antenna for receiving millimeter wave radiation by scanning afield of view using a first fan beam; a filter comprising at least onepolarization rotating transflector for rotating polarization of incidentmillimeter wave radiation through 90 degrees; and another fan beamantenna for receiving said polarization-rotated millimeter waveradiation by scanning a field of view using a second fan beam, said fanbeams intersecting and being geometrically orthogonal to each other, andsaid radiation being co-polarized, said fan beam antennas each being amodified pill-box antenna including: a metal housing with an elongatedaperture in at least one side of said housing; a curved primaryreflector surface located within said housing and opposite saidaperture; a feed horn within said housing; and one or moresub-reflectors for coupling said feed horn to said primary reflectorsurface, at least one of said sub-reflectors being designed to rotateand providing one-dimensional beam scanning in a narrow direction ofsaid other fan beam.
 17. The antenna according to claim 16, wherein saidpolarization rotating transreflector includes: a planar metallicreflector; and a grid of closely spaced wires, said grid spaced n×λ/4from said planar metallic reflector, where n is an odd integer and λ isa wavelength of said millimeter wave radiation.
 18. The antennaaccording to claim 17, wherein said polarization rotating transreflectoris positioned at a 45 degree angle relative to an aperture of said otherfan beam antenna and at a substantially 45 degree angle relative to thedirection of incident millimeter-wave radiation.
 19. An apparatus forforming an image from millimeter waves, said apparatus characterized byincluding: an antenna for receiving millimeter wave radiation accordingto claim 16; and a receiver for cross-correlating components of saidreceived millimeter wave radiation from said fan beams.
 20. Theapparatus according to claim 19, wherein polarizations of the electricfields of said two fan beams are substantially parallel in alignment.21. The apparatus according to claim 19, wherein scanning by saidantenna is performed in azimuth and elevation defining a scan range, andan intersection region of said two fan beams is able to cover any pointin said scan range.
 22. The apparatus according to claim 21, whereinsaid scan range determines said field of view and a beam width of eachfan-beam in a narrow direction determines an angular resolution of saidimage.
 23. The apparatus according to claim 19, further including aprocessor for measuring cross-correlated output dependent upon saidreceived millimeter wave radiation at each field point in said field ofview to produce a map of brightness.
 24. The apparatus according toclaim 23, further including a controller for controlling said twogeometrically orthogonal, intersecting fan beams to generate saidcross-correlated output at each fan beam intersection point in saidfield of view.
 25. An apparatus for receiving millimeter wave radiationfor generating an image, said apparatus including: an antenna forreceiving millimeter wave radiation according to claim 16; adownconverter for downconverting components of said millimeter waveradiation received in accordance with said first and second beams togenerate respective intermediate frequency (IF) signals; and acorrelator for cross-correlating said IF signals; and a filter forfiltering the resulting cross-correlated signal to provide a valueproportional to brightness at each point in the scene.
 26. The apparatusaccording to claim 16, further including an amplifier for amplifyingsaid received millimeter wave radiation in accordance with said firstand second beams prior to downconverting.
 27. A millimeter wave imagingsystem, including: an antenna for receiving millimeter wave radiationaccording to claim 16; a millimeter wave receiver coupled to saidantenna, including: first and second receivers respectively coupled tosaid fan beam antennas respectively for receiving millimeter waveradiation in accordance with said fan beams; downconverters fordownconverting respective components of said received millimeter waveradiation received from said first and second receivers to generaterespective intermediate frequency (IF) signals; a correlator forcross-correlating said IF signals; and a filter for filtering theresulting cross-correlated signal to produce a filtered,cross-correlated signal proportional to brightness in said field of viewas said antenna beams are scanned; and a processing unit for building upsaid image using said filtered, cross-correlated signal.
 28. The systemaccording to claim 27, wherein said processing unit independentlyproduces control signals for scanning said antenna, so that said imagecan be generated from said filtered, cross-correlated signal, whichprovides a value proportional to the brightness of the scene at eachpoint in the field of view.